Hydrogen may be used as a renewable fuel. For example, hydrogen may be produced by electrolysis, stored, and used as a fuel. Hydrogen fuel may be used to power fuel cells in automobile applications, for example. Storing of the hydrogen may present obstacles as hydrogen may pass through the walls of traditional high pressure gas tanks. Recently, solid materials, such as metals, have been used to chemically store and release hydrogen.
Metal hydride alloys are metal alloys that absorb and desorb hydrogen. Mg, Mg2Ni, FeTi are examples of metal hydride metals and alloys that absorb and desorb hydrogen. Hydrogen charging/discharging processes in metal hydrides, providing hydrogen storage, may have the characteristics of solid-state transformations. The kinetics of the transformation may depend on thermodynamic driving forces and nucleation barriers. For example, charging of hydrogen storage materials may necessitate the delivery of hydrogen from a gas phase to the interior of the material to form a hydride phase and discharging of hydrogen storage materials may necessitate the removal of hydrogen from the hydride phase to a gas phase.
Although the diffusion of hydrogen in most materials may be fast, the diffusion in metal hydrides may typically be slow and may be a limiting factor of the transformation. To overcome these limitations, the common practice has been to increase a solid/gas surface area and decrease the diffusion distance to the interior of the particle by refining particle size. For example, particle size refinement has been achieved by mechanical attrition (e.g., ball milling). Additives have also been used to facilitate the milling process and improve the dissociation of molecular hydrogen from surfaces of the metal hydride.
Recently, Mg-based hydrides have attracted attention in the development of solid-state hydrogen storage systems due to the high hydrogen content of MgH2, low cost of Mg, and single-step reaction path. However, challenges remain with the usage of Mg-based hydrides. For example, the use of Mg-based hydrides in hydrogen storage systems for fuel cells may cause high thermodynamic stability of the hydrogen storage system which may lead to excessively high operating temperatures (e.g. T≧673 K) and/or sluggish kinetics of the hydrogen storage system. Modification of the thermodynamic and kinetic properties of Mg-based hydrides has typically been focused on the powder/grain refinement and alloying/catalyzing of magnesium, predominately by ball milling. These approaches may not provide desirable operating temperatures and/or kinetics of the hydrogen storage systems in some applications.
What is needed are improved hydrogen storage systems and materials and methods of making the same.